The specific aim of this research program is to identify chemical alterations produced by ionizing radiation in the sugar moiety of mononucleotides and to assess the role of the base and phosphate in the production of these alterations. Ultimately this information will contribute to achieving the long term objective of characterizing chemical events which result in single strand breaks. The approach to this problem is to identify, by ESR and ENDOR, free radical products and reactions in single crystals of mononucleotides and other DNA model compounds. One of the specific objectives is to determine a comprehensive reaction scheme accounting for all major free radical products in both 3' cytidylic acid and 5' deoxycytidylic acid, x-irradiated and observed in the temperature range between 10K and 300K. Also, under study is aminoethanolphosphate in the form of single crystals. This compound serves as a model for studying the involvement of the phosphate in free radical damage at the phosphoester bond of nucleotides. A stable free radical, labeled 3 alpha H, is formed in 3' cytidylic acid single crystals by x-irradiating at 77K and then warming to above room temperature. It remains stable up to 150 degrees C. ESR and ENDOR measurements explicitly indicate a structure of the form H2C-CX=CYH. The same free radical structure has been observed in 5' deoxycytidylic acid as well. BIBLIOGRAPHIC REFERENCES: Close, D.M., Fouse, G.W., and Bernhard, W.A., ESR and ENDOR Study of Single Crystals of L-Asparagine H2O-Irradiated at Room Temperature, J. Chem. Phys. 66, 1534-1540 (1977). Close, D.M., Fouse, G.W., and Bernhard, W.A., ENDOR Study of the 3 alpha H Radical in Deoxycytidine 5'-Phosphate H2O, J. Chem. Phys. 66, 4689-4693 (1977).